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Abstract

Experimental campaigns at the Joint European Torus with an ITER-like Be/W wall (JET-ILW) with pure deuterium (D), tritium (T) and Deuterium-Tritium (DT) were a unique opportunity to explore various aspects related to the ITER operation. It should also be emphasized that the JET campaign at the end of 2021 included the first tokamak experiments using a 50-50 D-T fuel mixture since 1997. Therefore, one of the most important challenges in recent years was the development of the hybrid scenario for D-T, based on reference deuterium and tritium plasmas. This kind of scenario, one of the foreseen for ITER, is characterized by a low current plasma and a high normalized beta βN factor compared to the parallel optimised baseline scenario. On JET-ILW, the hybrid scenario is usually obtained by modifying the q-profile to help avoid MHD instabilities, and has an increased normalized confinement time compared to the H98,y2-scaling.

As the aforementioned experiments have shown, controlling the plasma edge in the different phases of the scenario becomes more difficult with higher isotope mass and therefore beware of uncontrolled impurity accumulation. For this reason, investigation of the impurity behaviour, as well as their control, constituted the crucial issue. Present contribution aims to compare mid-Z (e.g. Nickel (Ni), Copper (Cu), Iron (Fe), Molybdenum (Mo)) and high Z (Tungsten (W)) impurities behaviour within H-mode hybrid discharges in D and T plasmas, as well as D and DT plasmas. The presented results rely mainly on the measurements collected by VUV SPRED survey spectrometer and bolometry diagnostic. Detailed analysis shows that in H-mode regime in hybrid scenario, higher impurity radiation is observed for DT in comparison to D plasmas, as well as for T compared to D plasmas. Additionally, it was noticed that the most significant contribution to the plasma radiated power comes from tungsten and to a lesser extent from Ni (~10%). Moreover, it was found that earlier gas puff and earlier transition from small ELMs to ELM-free phase results in earlier increase of impurities and also that ICRH power in H-mode entrance phase have influence on Ni impurity behaviour.